This UCLA symposium will address the molecular aspects of myofibril assembly. The morphological term "striated muscle' derives from repeats of a basic structural unit, the sarcomere. Most of the known sarcomeric proteins have been cloned and partially sequenced. Thus, this tools for molecular analysis of assembly are in hand, as evidenced by studies using Drosophils, G. elegant and many vertebrate systems. Myogenesis is entering a new phase. Regulatory genes have now been identified including myoD, myogenin and myd which may encode trans-acting proteins involved in the expression of muscle-specific proteins. Most contractile proteins are expressed as multiple isoforms whose individual functional properties in myofibril assembly are unknown but are beginning to emerge. There is a reservoir of information on the physical-chemical properties of myosin and actin polymerization, but very little is known about the function of filament-associated proteins in theme assembly reactions or their interrelationships with cytoskeleton and plasma membrane components. Genetic approaches have now been demonstrated to be feasible, and critical questions about the biological assembly of myofibrils can now be addressed and resolved. This conference will pull together the leading investigators in the areas of protein polymerization, molecular genetics, and myogenesis, with the aim of focussing attention upon the major unanswered questions in the assembly of this essential organelle.